The long term goal of this project is to understand basic mechanisms of associative learning. Associative learning is the way in which causal relationships are extracted from the environment and is of great adaptive significance since it allows animals to modify their behavior on the basis of learned associations between cues and consequences. Two variables are important in associative learning: the close temporal association, or contiguity of the stimuli, and the reliability with which the first stimulus predicts the occurrence of the second. Although many neural correlates of contiguity have been described, no consensus yet exists as to the mechanism of associative learning. In particular, very little is known regarding the cellular basis of predictability. The work proposed here will use behavioral, cellular, pharmacological and biophysical techniques to investigate mechanisms of the associative process, with a special emphasis on predictability. The medicinal leech, Hirudo medicinalis is a favorable animal for such studies; its stereotyped, reiterated nervous system has permitted a detailed mapping of sensory and motor pathways at the level of single, identified cells. Leeches can be taught to associate a light touch (CS) with a shock (US). Several cells in the neural pathways mediating these stimuli have been identified and potential sites of convergence between the two have been found. The neurotransmitter serotonin and the peptide proctolin are implicated in the increased excitability correlated with learning and will be pursued further in the proposed experiments. Anatomical experiments will be conducted to identify the source of these compounds and the ionic basis of the increased excitability will be determined. Two specific hypotheses will be tested: 1) these compounds modulate background potassium (K+) channels, important determinants of a cell's excitable properties and 2) second messengers will mediate the channel modulation. Cell killing and 5HT depletion techniques will be used to determine the causal relationship of particular cells or transmitter systems to associative learning. Many fundamental cellular mechanisms were established before invertebrates and vertebrates diverged, and it is reasonable to expect that a process as basic as learning will show strong parallels in the two groups. Thus with an understanding of the basic behavioral and cellular mechanisms of learning, better therapies and pharmacological interventions can be developed which would be useful for the treatment of mental retardation and in the amelioration of the memory deficits often seen in aging.